Synthesis and photophysical properties of glass-forming bay-substituted perylenediimide derivatives

Article abstract of DOI:10.1021/jp907697f

A series of perylenediimide-based small molecules (PDI1-PDI5) containing electron-deficient groups in the bay region were synthesized and characterized. The PDI derivatives were found to be capable of forming molecular glasses with glass transition temperatures ranging from 50 to 102 ?°C. Detailed investigations of the optical properties of the synthesized derivatives were performed and compared with those obtained from quantum chemical calculations. Optimized molecular structures of the PDI derivatives exhibited core-twisting by 16?° and torsional angle between the bay substituent and the perylene core in the range of 60-72?°. The PDI derivatives exhibited absorption maxima in the range of 2.27-2.36 eV and emission maxima in the range of 2.10-2.28 eV. The impact of the bay substituents on the emission, fluorescence quantum yield, and lifetimes in solutions and thin films was established. The red shift of emission maxima (from 2.282 to 2.095 eV) observed for various PDIs in solutions was accompanied by significant reduction in the emission quantum yield (from 0.73 to 0.44) and corresponding increase of the fluorescence lifetime (from 4.5 to 6.8 ns). This was in agreement with quantum chemical calculations indicating decrease of the radiative relaxation rate due to reduction of the oscillator strength and remarkable decrease of the torsional activation barrier. The spectral properties of the wet-casted perylenediimide films featuring different bay substituents were also studied. The variation in the emission peak (of 0.25 eV) and the considerable increase of the Stokes shift (of 0.4 eV) are explained in terms of the formation of the amorphous state. The influence of the bay substituents on the thermal and spectral properties of the films are discussed. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp907697f

1782
J. Phys. Chem. B 2010, 114, 1782–1789
Synthesis and Photophysical Properties of Glass-Forming Bay-Substituted Perylenediimide
Derivatives
V. Sivamurugan,† K. Kazlauskas,‡ S. Jursenas,‡ A. Gruodis,§ J. Simokaitiene,|
J. V. Grazulevicius,| and S. Valiyaveettil*,†
S5-01-01, Materials Research Laboratory, Department of Chemistry, National UniVersity of Singapore,
3 Science DriVe 3, Singapore 117 543; Institute of Applied Research, Vilnius UniVersity, Saule¨tekio 9-III,
LT-10222 Vilnius, Lithuania; Department of General Physics and Spectroscopy, Vilnius UniVersity,
Saule¨tekio 9-III, LT-10222 Vilnius, Lithuania; and Department Organic Technology, Kaunas UniVersity of
Technology, RadVilenu pl. 19, LT-50254, Kaunas, Lithuania
ReceiVed: August 10, 2009; ReVised Manuscript ReceiVed: December 12, 2009
A series of perylenediimide-based small molecules (PDI1-PDI5) containing electron-deficient groups in the
bay region were synthesized and characterized. The PDI derivatives were found to be capable of forming
molecular glasses with glass transition temperatures ranging from 50 to 102 °C. Detailed investigations of
the optical properties of the synthesized derivatives were performed and compared with those obtained from
quantum chemical calculations. Optimized molecular structures of the PDI derivatives exhibited core-twisting
by 16° and torsional angle between the bay substituent and the perylene core in the range of 60-72°. The
PDI derivatives exhibited absorption maxima in the range of 2.27-2.36 eV and emission maxima in the
range of 2.10-2.28 eV. The impact of the bay substituents on the emission, fluorescence quantum yield, and
lifetimes in solutions and thin films was established. The red shift of emission maxima (from 2.282 to 2.095
eV) observed for various PDIs in solutions was accompanied by significant reduction in the emission quantum
yield (from 0.73 to 0.44) and corresponding increase of the fluorescence lifetime (from 4.5 to 6.8 ns). This
was in agreement with quantum chemical calculations indicating decrease of the radiative relaxation rate due
to reduction of the oscillator strength and remarkable decrease of the torsional activation barrier. The spectral
properties of the wet-casted perylenediimide films featuring different bay substituents were also studied. The
variation in the emission peak (of 0.25 eV) and the considerable increase of the Stokes shift (of 0.4 eV) are
explained in terms of the formation of the amorphous state. The influence of the bay substituents on the
thermal and spectral properties of the films are discussed.
Introduction
generally result in significant luminescence quenching. An
alternative way for tuning the absorption/emission wavelength
either by modification of the diimide groups or by substitution
at perylene core has been proposed.6 The modifications of the
diimide group via the ring opening of the imide moiety,
replacement of the carbonyl groups by imino groups and their
incorporation into a six-membered rings,6 alteration of the ring
sizes, etc. enabled reasonable spectral tuning through extended
conjugation and in some cases yielded luminescence efficiency
close to 100%. However, efforts in achieving amplified emission
from the derivatives with modified diimide group were unsuc-
cessful.7 PDI derivatives substituted at the bay positions of the
perylene core (Figure 1) using C-O, C-C, and C-N coupling
are considered to be promising,8 owing to the fact that the steric
hindrance and perylene core twisting induced by the bay substit-
uents could prevent π-π stacking,9 and thus reduce luminescence
quenching. Also the majority of perylene dyes favor H-type
aggregation with weak fluorescence,10 owing to the steric inhibition
of substituents. However, a few reports on the perylene core
substituted PDI derivatives, like tetrachloro-,11 cholesterol-,12 and
various aryl-substituted7,13 perylenes, showed reduced lumines-
cence quantum yield (<85%) as compared to those substituted
at the imide nitrogen. Recently, we have reported optical and
charge transport properties of core substituted perylenediimide
derivatives.14
Perylenediimides (PDIs) are widely investigated organic
materials with important applications in optoelectronics. Al-
though the key application of PDIs so far is in laser dyes,1 such
as the famous perylene orange and red dyes, other potential
applications of PDIs in organic field-effect transistors,2 photo-
voltaic cells,3 and light-emitting diodes4 are being explored.
Chemical, thermal stability, and photostability accompanied by
high luminescence efficiency and good solution processability5,6
make optoelectronic applications of PDIs viable. It is interesting
to note that the high photostability of PDIs as compared to other
organic compounds is advantageous for optoelectronic applica-
tions, in particular, for laser applications.
For PDI-based light-emitting devices, it is essential to provide
tunability of the emission wavelength. Although the imide
nitrogen substituted PDI derivatives demonstrate excellent
thermal and photostability, and show superior luminescence
quantum yield (QY) (up to 100%), these molecules have
negligible spectral tunability.6 Additionally, the planar structure
of perylenediimide core facilitates the formation of H aggregates
(face to face) in the concentrated solutions and solid state, which
* Corresponding author. E mail: chmsv@nus.edu.sg. Fax: (+65) 67791691.
Phone: (+65) 65164327.
† National University of Singapore.
‡ Institute of Applied Research, Vilnius University.
§ Department of General Physics and Spectroscopy, Vilnius University.
| Kaunas University of Technology.
Here we report the luminescence properties of a series of
perylenediimide derivatives with electron accepting substituents
10.1021/jp907697f  2010 American Chemical Society
Published on Web 01/20/2010

Bay-Substituted Perylenediimide Derivatives
J. Phys. Chem. B, Vol. 114, No. 5, 2010 1783
Figure 1. Chemical structure of PDI derivatives.
Figure 3. Calculated total energy of the ground state S0 and the first
excited state S1 (a) and oscillator strength for the transition S0 f S1
(b) as a function of dihedral angle ꢀ (at fixed angle R, R ) 16°) for
PDI2 derivative. Dashed line designates energy minimum of S1 state.
Solid lines are the guides for the eye.
dipentafluorophenoxy substituents attached to the bay positions
of PDIs.16 Literature reported core-twist angle for the bromo-
substituted PDI (the case of PDI1) and calculated by semiem-
pirical AM1 method is somewhat larger (21°).17 The dihedral
angle ꢀ formed by the planes of the bay substituent and half-
unit of the core is determined to be substituent dependent.
Quantum chemical calculations also revealed an insignificant
8° inclination of the long axis of the bay substituents with respect
to the half-units of the core.
Figure 2. Molecular structure of the bay-substituted PDI derivative,
PDI5. R, dihedral angle between the two subplanes (half-units) of the
PDI core; ꢀ, dihedral angle between the planes of the bay substituent
and half-unit of the core.
The differences in the state energies and oscillator strengths
of the two selected PDI derivatives, PDI2 and PDI5, imposed
by the torsional motions of their bay substituents are revealed.
The selection of the representative derivative PDI2 from the
group of PDI1-PDI2 and the derivative PDI5 from the group
of PDI3-PDI5 having nitrogen in the bay substituents was based
on the pronounced difference in the optical properties (spectral
positions of the fluorescence and absorption bands, fluorescence
quantum yields) of these groups, which are discussed below.
Figure 3 demonstrates total energies of the ground state S0
and the first excited state S1 as well as the oscillator strength
for the transition S0 f S1 as a function of dihedral angle ꢀ
calculated for the PDI2 molecule. Since the attachment of
different bay substituents resulted in the similarly twisted
perylene core for PDI2-PDI5, calculations are presented for
the fixed dihedral angle R. For comparison, an analogous figure
showing ꢀ angle dependences of the state energies and oscillator
strength for PDI5 molecule is displayed in Figure 4.
Energies of the S0 and S1 states in PDI2 molecule exhibit
minima at the same dihedral angle ꢀ (ꢀ ) 72°), indicating
unchanged orientation of 2,4-di(trifluoromethyl)phenyl substitu-
ent upon transition from the ground to the first excited state.
The energy minima of S0 and S1 states in PDI5 correspond to
different ꢀ angles, 70° and 60°, respectively, implying torsion
of 4-pyridyl substituent plane toward the core plane upon
excitation. Very similar torsions (by ∼10°) of the bay substit-
uents were also revealed for PDI3 and PDI4 molecules upon
their excitation. Deeper energy minima obtained for PDI2 as
compared to that for PDI5 (see Figures 3a and 4a) indicate
higher torsional activation barrier needed for the bay substituent
to accomplish this motion. High torsional activation barrier can
significantly affect optical properties of the molecules and
obstruct close packing of the molecules in the films.
at the bay positions of the perylene core (Figure 1). By
estimating quantum yield (QY) and luminescence decay time,
the connection between the (non) radiative relaxation rates and
the QY of the PDI derivative with respect to the bay substituent
was established. This allows better understanding of the
substituent-induced luminescence properties and assists in
selecting the PDI derivatives for further applications.
DFT Calculations. Quantum chemical calculations of the PDI
derivatives were carried out using density functional theory
(DFT) as implemented in the Gaussian 0315 software package.
Molecular structure of the ground electronic state was optimized
using B3LYP functional theory with the 6-311G basis set.
Excited-state energies and oscillator strengths for singlet transi-
tions were obtained by means of the ZINDO procedure. The
optimized molecular structure of the bay-substituted PDI
derivatives achieved via total energy minimization revealed two
important structure distortions which could be attributed to all
PDI molecules studied, in particular, twisted perylene core as
well as the twisted bay substituent around the methyl bridge
connecting it to the core, in accordance with previous reports.16,17
The twist planes and angles for one of the PDI derivatives, PDI5,
are illustrated in Figure 2.
It is known that PDIs with twisted core have important
implications for their arrangement in well-organized supramo-
lecular structures with controlled optical and electronic proper-
ties, and thus, are considered to be promising for organic
electronics.16,17
Our calculations imply that the dihedral angle R between the
two subplanes of the core is similar (R ) 16°) for PDI2-PDI5
derivatives, i.e., independent of the substituents at the bay region.
The determined dihedral angle is in good agreement with the
angle 15-18° reported in the literature for diphenoxy and

1784 J. Phys. Chem. B, Vol. 114, No. 5, 2010
Sivamurugan et al.
Figure 4. Calculated total energy of the ground state S0 and the first
excited state S1 (a) and oscillator strength for the transition S0 f S1
(b) as a function of dihedral angle ꢀ (at fixed angle R, R ) 16°) for
PDI5 derivative. Dashed line designates energy minimum of S1 state.
Solid lines are the guides for the eye.
Calculated S0 f S1 transition energies for PDI2 and PDI5
derivatives are 2.27 and 2.2 eV, respectively. Estimate of the
oscillator strength for the transition S0 f S1 in PDI2 as predicted
by quantum chemical calculations is 0.92, whereas, his estimate
for PDI5 molecule is 0.73 (dashed lines in Figures 3b and 4b).
Similar oscillator strengths of the order of 0.7 were also obtained
for PDI3 and PDI4 molecules with nitrogen atoms in the bay
substituents. Theory predicted 1.3 times lower oscillator strengths
for the PDI3-PDI5 derivatives as compared to that for PDI2
and indicated diminished absorbance and fluorescence quantum
yield.
Figure 5 illustrates a change of the molecular charge density
and the main directions of this change corresponding to the S0
f S1 transition in representative PDI2 and PDI5 molecules.
Obviously, upon excitation the charge redistribution in both
molecules occurs only within the half units of the perylene core
due to its twisted conformation. In PDI2 molecule 2,4-
di(trifluoromethyl)phenyl substituents attached to the bay posi-
tion of the core act as strong electron acceptors, and thus, the
electron cloud is attracted toward the substituents (see Figure
5a). Meanwhile, in PDI5 molecule (Figure 5b) as well as in
other PDI3 and PDI4 molecules, the charge redistribution is
more limited to the half units of the core without marked
influence of the bay substituents.
Figure 5. Redistribution of molecular charge density corresponding
to the S0 f S1 transition calculated for PDI2 (a) and PDI5 (b). Open
circles indicate a decrease, filled circles an increase of the molecular
charge. The big circles correspond to the amount of the charge density
changed. Arrows show main directions of molecular charge redistribution.
air. Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements were performed on TA
Instruments Q100 (heating rate 20 °C. min-1 for TGA and
heating/cooling rate of 10 °C. min-1 for DSC measurements)
using nitrogen as carrier gas.
Fluorescence Kinetics and Quantum Yield Studies. PDI
derivatives were excited using light-emitting diode (Nichia
NSPB500AS) coupled to monochromator to produce emission
at 490 nm with a full width at half-maximum of 13 nm.
Photoluminescence (PL) spectra were measured using back-
thinned CCD spectrophotometer PMA-11 (Hamamatsu) and
corrected for the wavelength-dependent sensitivity of the CCD.
Fluorescein in 0.1 M NaOH aqueous solution with QY of 0.92
has been used as a reference.18 The QY of the PDI solutions
was estimated by comparing wavelength-integrated PL intensity
of the solution with that of the reference. Optical densities of
the reference and solutions of the PDI derivatives were kept
below 0.05 to avoid reabsorption effects. Estimated QY was
verified by using an alternative method of an integrating sphere
(Sphere Optics) coupled to the CCD spectrometer.19,20 The
integrated sphere method was also employed in the QY
estimations of the PDI films.
Experimental Section
Materials and Methods. All chemicals and reagents were
purchased from Aldrich and used as received. All reactions were
done in distilled anhydrous solvents under inert atmosphere.
The NMR spectral data of all synthesized samples were recorded
on a Bruker ACF 300 spectrometer operating at 300 and 75.5
1
MHz for H and 13C nuclei, respectively, using CDCl3 solvent
and TMS as internal standard. The absorbance and fluorescence
spectra were recorded on a Shimadzu 3101 PC spectrophotom-
eter and RF-5301PC Shimadzu spectrofluorometer, respectively.
Tetrahydrofuran solutions of the PDI derivatives (concentration
1 × 10-5 M) in 10 × 10 mm3 quartz cells were used for
luminescence measurements. Neat films of the PDI derivatives
used in the investigations were prepared from 1 × 10-3 M THF
solutions on the glass substrates by drop-casting in an ambient
PL decay curves of the PDI derivatives were measured using
time-correlated single-photon-counting system PicoHarp 300
(PicoQuant GmbH). Pulsed excitation at 1 MHz repetition rate

Bay-Substituted Perylenediimide Derivatives
J. Phys. Chem. B, Vol. 114, No. 5, 2010 1785
was provided by picosecond diode laser with the pulse duration
of 70 ps and the emission wavelength of 375 nm.
SCHEME 1: Synthesis of Perylenediimide Derivatives:
(i) R-B(OH)2, Pd(PPh3)4, 1M K2CO3, THF, N2, 75°C
Synthesis of 1,7-Dibromo-N,N′-didodecylperylenediim-
ide (PDI1). Synthesis and characterization of 1,7-dibromo-
perylenedianhydride and 1,7-dibromoperylenediimide were re-
ported earlier.14
Synthesis of Perylenediimide Derivatives (PDI2-5). PDI1
(1.12 mmol) and aryl monoboronic acid (2.8 mmol) were
dissolved in 50 mL of THF (AR grade) and heated to 70 °C for
15 min under nitrogen atmosphere. About 20 mL of 1 M K2CO3
aqueous solution and 10 mol % Pd(PPh3)4 was added to the
mixture. The contents were heated at 75 °C under nitrogen
atmosphere for 20-24 h. After completion, the reaction mixture
was diluted with ethyl acetate (300 mL) and extracted with water
(3 × 300 mL). The crude product was purified by column
chromatography using a mixture of 20% ethyl acetate and 80%
hexane as eluent and silicagel (200-400 mesh) as stationary
phase.
TABLE 1: Thermal Characteristics of Perylenediimide
Derivatives
PDI
derivatives
Tm (°C)
Tg (°C)
Tcr (°C)
TID-5% (°C)
N,N′-Bis(dodecyl)-1,7-bis(2,4-di(trifluoromethyl)phenyl)-
perylene-3,4,9,10-diimide (PDI2). Yield: 75% (0.75 g); Red
orange solid. 1H NMR (300 MHz, CDCl3, δ ppm) 8.52 (s, 2H,
Per-H), 8.21-8.27 (m, 4H, Per-H and Ar-H), 7.97-8.00 (m,
2H, Per-H), 7.54-7.57 (d, 4H, Ar-H), 4.18-4.13 (t, 4H,
N-CH2), 1.82-1.77 (m, 6H, alkyl-CH2), 1.39-1.25 (m, 24H,
alkyl-CH2), 0.88 - 0.64 (m, 12H, alkyl-CH3). 13C NMR (75
MHz, CDCl3, δ ppm) 163.2, 163.0, 154.9, 140.8 138.9, 137.2,
134.7, 134.1, 129.9, 128.9, 128.7, 127.1, 122.0, 121.7, 105.9,
77.4, 77.0, 76.5, 61.2, 56.6, 40.8, 31.9, 29.6, 29.5, 29.4, 29.3,
28.1, 27.2, 22.6, 14.0. EI-MS m/z: 1150.72 (Exact Mass:
1150.45). Anal. Calcd for C64H62F12N2O4: C, 66.77; H, 5.43;
F, 19.80; N, 2.64; O, 15.10. Found. C, 66.73; H, 5.37; F, 19.86;
N, 2.38.
PDI1
PDI2
PDI3
PDI4
PDI5
155
-
50
57
99
102
81
-
-
436
443
460
443
450
245
-
155
-
-
-
1.72-1.63 (m, 8H, alkyl-CH2-), 1.44-1.29 (m, 30H, alkyl-CH2),
0.92-0.84 (m, 12H, alkyl-CH3). 13C NMR (75 MHz, CDCl3, δ
ppm) 162.8, 134.3, 132.4, 130.1, 128.8, 128.0, 124.2, 123.05,
122.7, 77.4, 77.0, 76.5, 40.6, 31.9, 29.6, 29.6, 29.5, 29.3, 28.1,
27.1, 22.6, 22.5, 14.1. EI-MS m/z: 880.67 (Exact Mass: 880.49).
Anal. Calcd for C58H64N4O4: C, 79.06; H, 7.32; N, 6.36; O,
7.26. Found: C, 78.85; H, 7.26; N, 6.05.
N,N′-Bis(dodecyl)-1,7-bis(4-cyanophenyl)perylene-3,4,9,10-
diimide (PDI3). Yield: 75% (0.7 g); Red color solid. 1H NMR
(300 MHz, CDCl3, δ ppm) 8.64 (s, 2H, Per-H), 8.20-8.18 (d,
2H, Per-H), 7.83-7.79 (m, 4H, Ar-H), 7.71-7.67 (m, 6H,
Per-H and Ar-H), 7.31-7.30 (d, 2H, Ar-H), 7.20-7.17 (t,
2H, Tp-H), 4.20-4.15 (t, 4H, N-CH2), 1.80-1.65 (m, 4H,
alkyl-CH2-), 1.45-1.30 (m, 26H, alkyl-CH2), 0.88-0.65 (m,
8H, alkyl-CH3). 13C NMR (75 MHz, CDCl3, δ ppm) 163.0,
162.9, 146.5, 138.9, 134.6, 133.9, 132.4, 130.7, 130.0, 129.8,
129.0, 127.9, 122.9, 122.6, 118.0, 112.8, 77.4, 77.0, 76.5, 40.7,
31.9, 29.6, 29.5, 29.34, 29.3, 28.1, 27.0, 22.6, 14.0. EI-MS: m/z
928.02 (Exact Mass: 928.49). Anal. Calcd for C62H64N2O4: C,
80.14; H, 6.94; N, 6.03; O, 6.89. Found: C, 80.32; H, 6.97; N,
5.78.
N,N′-Bis(dodecyl)-1,7-bis(3-nitrophenyl)perylene-3,4,9,10-
diimide (PDI4). Yield: 80% (0.77 g); Red solid. 1H MNR (300
MHz, CDCl3, δ ppm) 8.61 (s, 2H, Per-H), 8.59-8.35 (m, 4H,
Per-H and Ar-H), 8.20-8.15 (d, 2H, Per-H), 7.94-7.92 (d,
2H, Per-H), 7.77-7.67 (m, 4H, Ar-H), 4.19-4.14 (t, 4H,
N-CH2), 1.74-1.65 (m, 6H, alkyl-CH2-), 1.52-1.24 (m, 30H,
alkyl-CH2), 0.88-0.80 (m, 8H, alkyl-CH3). 13C NMR (75 MHz,
CDCl3, δ ppm) 163.5, 163.4, 158.7, 141.0, 137.2, 135.6, 134.4,
130.1, 129.9, 129.3, 128.0, 127.3, 122.3, 121.8, 119.7, 106.0,
55.4, 40.6, 31.9, 29.6, 29.5, 29.4, 29.3, 28.1, 27.1, 22.6, 14.0,
-0.0. EI-MS m/z: 968.17 (Exact Mass: 968.47). Anal. Calcd
for C60H64N4O8: C, 74.36; H, 6.66; N, 5.78; O, 13.21. Found:
C, 74.22; H, 6.76; N, 5.38.
Results and Discussion
Synthesis of Perylenediimide Derivatives. The chemical
structures of perylenediimide derivatives (PDI1-PDI5) are shown
in Figure 1. The electron-deficient substituents such as bromo
(PDI1), 2,4-di(trifluoromethyl)phenyl (PDI2), 4-cyanophenyl
(PDI3), 3-nitrophenyl (PDI4), and 4-pyridyl (PDI5) were
incorporated in the bay region of perylenediimide core.
The optical behavior of PDI2-PDI5 were compared with 1,7-
dibromoperylenediimide (PDI1) and the effect of substituents
were studied. The PDI derivatives were synthesized using
Suzuki coupling in 70-80% yield and illustrated in Scheme
1.13,14 PDI1 was synthesized using bromination of perylene
dianhydride followed by condensation with n-dodecylamine.14a
PDI1 was purified by column chromatography (20% ethyl
acetate:hexane) followed by repeated washing with dichlo-
romethane:methanol (1:1) mixture and the pure 1,7-dibromoPDI
was obtained in 65% yield. The PDI derivatives were character-
1
ized using H and 13CNMR (see Supporting Information), EI-
MS, and elemental analysis.
Thermal Properties. Thermal stability, glass transition, and
crystallization behavior of PDI derivatives were studied using
TGA and DSC techniques, and the results are summarized in
Table 1. TGA traces were recorded at a heating rate of 20 °C/
min under nitrogen atmosphere and are presented in Figure 6.
The substituted PDI2-PDI5 derivatives showed higher thermal
stability than PDI1, which might be due to increase in rigidity
after incorporation of bay substituents. The perylene derivatives
were found to be thermally stable up to ca. 440 °C with 5%
weight loss temperature (TID) ranging from 436 to 460 °C. The
first major weight loss observed from 450 to 500 °C might be
due to loss of two n-dodecyl chains (2 × C12H25). Irrespective
N,N′-Bis(dodecyl)-1,7-bis(4-pyridyl)perylene-3,4,9,10-diim-
1
ide (PDI5). Yield: 85% (0.7 g); Black color solid. H NMR
(300 MHz, CDCl3, δ ppm) 8.85-8.70 (m, 4H, Pyr-H), 8.55 (s,
2H, Per-H), 8.20-8.30 (m, 2H, Per-H), 7.78-7.75 (d, 2H, Per-
H), 7.65-7.60 (d, 4H, Pyr-H), 4.22-4.15 (t, 4H, N-CH2),

1786 J. Phys. Chem. B, Vol. 114, No. 5, 2010
Sivamurugan et al.
Figure 6. TGA traces of PDI derivatives recorded at a heating rate of
20 °C min-1 under nitrogen atmosphere.
Figure 8. Absorption (dashed line) and PL (solid line) spectra of the
PDI derivatives in chloroform.
Figure 7. DSC curves of (A) PDI1, PDI4, and PDI5, (B) PDI2 (dotted
line) and PDI3 (solid line). Heating rate: 10 °C min-1 under nitrogen
atmosphere.
Figure 9. Absorption (dashed line) and PL (solid line) spectra of the
films of PDI derivatives.
of the substitutents in the bay region, all the PDI derivatives
showed similar thermal decomposition curves.14a Values of Tg,
Tcr, and Tm of PDI derivatives were obtained from DSC
thermogram and are recorded at a heating rate of 10 °C/min
and summarized in Table 1. All DSC curves are presented in
Figure 7, A and B. Glass transition, melting, and crystallization
temperature were recorded during second heating and cooling
cycle, respectively. The DSC curves showed that all perylene-
diimide derivatives were amorphous materials, except PDI3. The
typical DSC curves for PDI2 and PDI3 are shown in Figure
7B. PDI2 which was obtained as an amorphous compound
showed only glass transition at 57 °C in the DSC scans. PDI3
showed endothermic melting signal at 245 °C in the first DSC
heating scan. During the slow cooling scan PDI3 did not show
recrystallization. It was transformed into amorphous (glassy)
state and in the second heating scan a glass transition at 99 °C
was observed. However, further heating revealed an exothermic
recrystallization peak with a maximum at 155 °C and an
endothermic melting peak with a maximum at 245 °C.
(QY), PL decay time constants (τ), and CIE 1931 color
coordinates of the PDI1-PDI5 derivatives are summarized in
Table 2.
The introduction of electron-withdrawing groups induces
rather small changes to the shape and maxima of the absorption
spectra of the PDI derivative solutions as compared to the
changes imposed by the electron-donating groups.14 This
observation can be explained by the lack of charge transfer from
electron-withdrawing substituents to the perylene core.8,9,14 The
main absorption peak energies of the PDI1 (2.36 eV) and PDI2
(2.37 eV) derivatives are similar to those of PDIs with
symmetrical substitution at the imide nitrogen positions and can
be attributed to the π-π* transition in the perylene core.13 In
accordance with our theoretical calculations, the absorption band
of PDI3-PDI5 derivatives is red-shifted by ca. 0.07 eV as
compared to that of PDI1-PDI2. The main absorption peaks
of the PDI2 (2.37 eV) and PDI5 (2.3 eV) derivatives were found
to be 0.1 eV larger as compared to the calculated S0 f S1
transition energies for PDI2 (2.27 eV) and PDI5 (2.2 eV)
derivatives. This discrepancy between the experiment and
theoretical calculations is apparently caused by the solvation,
which was not taken into account in the calculations. The
absorption and PL spectra of the PDI derivatives are found to
be nearly mirror images of each other with clearly resolved
Spectral Properties. The spectral properties of the PDI
derivatives were investigated using UV-vis and PL spectros-
copy. The absorption and emission spectra of the PDI solutions
shown in Figure 8 were recorded in chloroform at a concentra-
tion of 10-5 M. The absorption and emission spectra of the PDI
films are displayed in Figure 9. The peak energies of the
absorption (Eabs) and emission (Eem) spectra, PL quantum yields

Bay-Substituted Perylenediimide Derivatives
J. Phys. Chem. B, Vol. 114, No. 5, 2010 1787
TABLE 2: Optical Properties of PDIs in Solution and Solid State
solution state
PDI
solid state
derivatives
Eabs (eV)
Eem (eV) QY τ (ns)
CIE (x,y)
Eabs (eV)
Eem (eV)
QY
τinitial (ns)
CIE (x,y)
PDI1
PDI2
PDI3
PDI4
PDI5
2.361, 2.530, 2.705
2.368, 2.537, 2.71
2.272, 2.422, 2.633
2.282, 2.439, 2.645
2.301, 2.452, 2.66
2.282
2.268
2.095
2.121
2.149
0.73
0.61
0.44
0.45
0.47
4.45
4.66
6.80
6.31
6.00
0.496, 0.481 2.126, 2.460
0.521, 0.465 2.342, 2.510
0.641, 0.647 2.153, 2.333
0.622, 0.366 2.205, 2.372
0.661, 0.324 2.225, 2.402
1.994
1.994
1.748
1.748
1.748
<0.005
<0.005
<0.005
<0.005
<0.005
<0.07
<0.07
<0.07
<0.07
<0.07
0.664, 0.336
0.606, 0.394
0.651, 0.349
0.673, 0.327
0.637, 0.363
vibronic structures, in particular, for the PDI1 and PDI2
derivatives. A pronounced progression of the vibronic peaks
with the strongest π-π* transitions is commonly observed in
perylenediimides due to their rigid core structure.11
PDI1-PDI2 solutions correspond to the yellow region, whereas
coordinates of PDI3-PDI5 solutions to the reddish-orange
region. The color coordinates of all the PDI films studied
correspond to the reddish-orange region, except the PDI2 film
with coordinates matching the orange region. Fluorescence
image of the thin films of PDI2, PDI3, and PDI4 is given in
the Supporting Information (Figure S1).
Absorption and PL band broadening results in an obscured
vibronic structure for PDI3-PDI5 solutions and can be caused
either by increased conjugation between the substituents and
the PDI core12 or by the substituent-induced twisting of the
core.14,21 Our quantum chemical calculations indicate the pos-
sible increase of conjugation between the substituents and the
core for PDI3-PDI5 due to the reduction of torsional angle
between the planes of the core and substituents. However, the
calculations also indicate that the substituent-induced twisting
of the core remains almost unchanged for all the PDIs studied.
Introduction of different substituents at the bay position of
the PDI core results in the red shift of the absorption maximum
from 2.36 to 2.27 eV and twice as large red shift of the PL
maximum from 2.28 to 2.10 eV. The pronounced bathochromic
shift of the emission maxima of the PDI3-PDI5 as compared
to the PDI1 and PDI2 by 0.17 eV is also in agreement with our
theoretical predictions that indicate lower S0 f S1 transition
energies as well as lower torsional activation barrier for
PDI3-PDI5. Continuous decrease of the PL quantum yield
following the red shift of the PL peak is observed in the PDI
derivatives. The origin of such behavior is caused by the
interplay between radiative and nonradiative relaxation pro-
cesses, which are discussed below.
Absorption and PL spectra of the PDI1-PDI5 films are
shown in Figure 9. The spectra of the films are broadened and
red-shifted as compared to that of the solutions. Despite the
spectral broadening, the persistent vibronic modes in the ab-
sorption spectra of the films closely resemble those in the
absorption spectra of dilute PDI solutions, where molecules are
considered as noninteracting. This suggests amorphous state of
the films of PDI2-PDI5 composed of the randomly oriented
molecules. Large torsion angles (∼70°) between the planes of
the perylene core and aromatic bay substituents revealed by the
calculations inhibit close packing of the PDI2-PDI5 molecules,
thus preventing their aggregation. An exception is the PDI1 film
composed of the molecules with the comparatively small 1,7-
dibromo substituents, which allow much closer packing of the
molecules. This is evidenced by the appearance of a new red-
shifted absorption band at 2.126 eV associated with aggregates.
Another distinctive feature is the 2-4 times larger Stokes shift
observed in the PDI films as compared to that in the solutions.
This is typical for amorphous systems, where owing to exciton
migration, lowest energy states have larger impact on the
emission properties. Drastically reduced emission efficiency of
the PDI films (QY < 0.005) as compared to the solutions is
attributed to enhanced exciton migration to quenching sites
associated with molecular oxygen unintentionally introduced in
the PDI films during their preparation in ambient conditions.
CIE 1931 chromaticity coordinates (x,y) of the PDI solutions
and thin films measured using KonicaMinolta CS-100A chro-
mameter are shown in Figure 10. The color coordinates of
Photoluminescence Decay Study. PL transients provide
information on the dominating excited-state relaxation processes.
Figure 11 depicts PL transients measured at the PL band maxima
of the dilute chloroform solutions of the substituted PDI
derivatives. As revealed by fitting of the experimental data, the
curves display mainly single-exponential decays with time
constants (τ) in the range of 4.5-6.8 ns with respect to the
substituents. PDI5 containing pyridyl substituents exhibits
double-exponential decay with the dominant relaxation time (τ2)
of 6.0 ns contributing up to 92% of overall PL intensity decay.
The origin of the minor decay component (τ1) with the fractional
intensity of only 8% is unclear; however, it might result from
insignificantly small traces of PDI1 used in the synthesis of
PDI5, and due to the spectral overlap of the PL bands of PDI1
and PDI5. The PDI derivative containing bromine (PDI1)
showed the fastest PL decay (τ ) 4.5 ns), whereas the derivative
with benzonitrile (PDI3) showed the slowest decay (τ ) 6.8
ns). PL decay time is found to be longer for the PDI derivative
solutions exhibiting larger Stokes shift.
PL decay curves measured in the neat PDI films bear
multiexponential character with the initial dominant decay times
of at least ∼102 shorter as compared to those in the solutions.
Since the shortest τ that can be reliably determined using our
photon-counting setup is limited roughly by the pulse width of
Figure 10. CIE 1931 xy chromaticity diagram of the PDIs in solution
(circles) and in the solid state (squares).

1788 J. Phys. Chem. B, Vol. 114, No. 5, 2010
Sivamurugan et al.
Figure 13. Radiative (squares), nonradiative (triangles), and measured
(circles) PL decay time as a function of PL quantum yield for a series
of PDI derivatives containing different substituents in chloroform
solution (10-5 M). Lines are guides for the eye.
bromine-substituted derivative (QY ) 0.73) and can be ac-
counted for by enhanced nonradiative relaxation. Increased
Stokes losses can be attributed to the lower torsional activation
barrier for the aromatic bay substituents in PDI3-PDI5 deriva-
tives as confirmed by quantum chemical calculations. A
complete domination of nonradiative decay processes for this
series of PDI derivatives decreases the decay time with increase
of the Stokes shift, and thus behavior similar to that of QY
should be observed. However, the opposite behaviors of the
PL decay time together with high PL QYs point out importance
of the radiative decay processes.
In order to unveil the competition of radiative and nonradia-
tive decay processes and explain an increase of the measured τ
with the Stokes shift, radiative (τR) and nonradiative (τN) decay
time constants for each of the PDI derivative was estimated by
using the following relations
Figure 11. PL transients (points) of the PDI derivatives in chloroform.
Lines represent exponential fits of the measured curves with decay time
constants τ.
QY ) τ/τR
-1
-1
τ-1 ) τR + τN
-1
Here the term τN takes into account of all possible
nonradiative decay pathways including intersystem crossing to
triplet states. The deduced τR and τN are plotted in Figure 13 as
a function of the PL QY, which represent a particular PDI
derivative with different bay substituents. Obviously, attach-
ments of various substituents at the bay positions of the perylene
core induce more rapid changes in radiative decay rate as
compared to that of nonradiative one. In particular, 10-fold
decrease in radiative decay (τR) and only 3-fold increase in
nonradiative decay (τN) followed by an increase of PL QY from
0.44 to 0.73, was deduced for this series of PDI derivatives
(see Figure 13). This plot unambiguously demonstrates the
governing role of radiative decay in the bromine (PDI1), 2,4-
bis(trifluoromethyl)benzene (PDI2), pyridine (PDI5), nitroben-
zene (PDI4), and benzonitrile (PDI3) substituted PDI derivative
solutions, which is responsible for increase of the measured τ
with the Stokes shift and behaves oppositely to the PL QY (see
Figure 13).
The larger variation of τR as compared to that of τN induced
by the attachment of different substituents at the perylene core
is most likely determined by the interaction between the
π-systems of the core and the substituents. Experimentally
estimated reduction of the radiative decay rate in PDI5 as
compared to that in PDI2 is in good agreement with the
weakening of the oscillator strength as predicted by our
calculations. Meanwhile, the tendency of an increase of non-
radiative decay rate in PDI1-PDI5 can be attributed to an
increase of torsional motion of the bay substituents. This
Figure 12. PL quantum yield (squares) and decay time (circles) of
10-5 M chloroform solutions of the PDI derivatives containing different
substituents as a function of Stokes shift. Lines are guides for the eye.
the excitation source, the initial τ in the films were shorter than
0.07 ns, and thus the decay closely followed the excitation pulse.
The multiexponential (or nonexponential) PL decay in the films
is usually explained by excitation delocalization over the
neighboring interacting chromophores followed by excitation
migration toward lower energy sites and subsequent emission
of photons. Exciton migration increased the probability of
collisional quenching with molecular oxygen thus causing the
dramatic reduction in fluorescence efficiency of the films.
In Figure 12, PL QY and decay time as functions of Stokes
shift are shown for the PDI derivatives. Decreased fluorescence
quantum yield from 0.61 in PDI2 to 0.47 in PDI5 agrees well
with the 1.3 times lower oscillator strength obtained from the
calculations for the transition S0 f S1 in PDI5 as compared to
that in PDI2. A clear tendency of decreasing QY and simulta-
neous increasing of the decay time followed by the increasing
Stokes shift for the PDI derivatives can be observed. Usually
increased energy (Stokes) losses indicate enhanced probability
for the excitation to relax via nonradiative pathways. Thus, about
twice as large Stokes shifts observed for pyridine-, nitroben-
zene-, and benzonitrile-substituted PDI derivative solutions result
in significantly lower QYs (∼0.45) as compared to that of

Bay-Substituted Perylenediimide Derivatives
J. Phys. Chem. B, Vol. 114, No. 5, 2010 1789
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The small perturbation of the core by 2,4-bis(trifluorometh-
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and aromatic substituents have been observed for other 1,7-
diaryl-substituted PDIs.13 The significant red shift in absorption
and PL spectra, and low PL QYs of pyridyl-, nitrophenyl-, and
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to the presence of nonbonded electron in the bay region and
lower torsional activation barrier of the bay substituents.
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Conclusion
A series of perylenediimide derivatives substituted at the bay
positions of the perylene core have been synthesized in good
yields. The new derivatives demonstrated glass-forming proper-
ties with the glass transition temperatures ranging from 57 to
102 °C. Photophysical studies indicated high luminescence
quantum yield in the solution (0.44-0.73). The bay substitution
induced red shift of the emission wavelength (of 0.18 eV) in
solution, which was followed by the increase of the Stokes shift
(0.1 eV) and 1.5-fold increase in the fluorescence lifetime.
Analysis of the luminescence lifetime and quantum yield
indicated that the substitution at the bay positions of the perylene
core invoked drastic changes in radiative decay rate and
relatively small ones in nonradiative decay rate. The large
variation of radiative relaxation rate was attributed to the
orientation-dependent interaction of the π-systems of the core
and the substituents due to potential steric effects at the bay
position, as was confirmed by quantum chemical calculations.
The spectral properties of the wet-casted perylenediimide
films resembled those of the molecules. Additionally, the larger
variation in the emission peak (of 0.25 eV) and considerable
increase of the Stokes shift (of 0.4 eV) implied formation of
the amorphous (glassy) state, which was also confirmed by
differential scanning calorimetry measurements.
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Acknowledgment. The financial support received from
Agency for Science, Technology and Research (A*STAR)
Singapore is gratefully acknowledged by the authors. The
research was supported by the Lithuanian State Science and
Studies Foundation. Department of Chemistry of National
University of Singapore is also acknowledged for technical
support.
1
Supporting Information Available: H and 13C NMR, and
MS spectra of all synthesized PDI derivatives are given. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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References and Notes
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JP907697F